1. Field of the Invention
This invention relates generally to telecommunications equipment, and particularly to a device and method for use in a Code Division Multiple Access (CDMA) telecommunications system.
2. Description of the Related Art
Code Division Multiple Access (CDMA) transmission schemes have become increasingly popular due to the recent growth of the wireless industry. CDMA is a spread spectrum technique whereby data signals are modulated by a pseudo-random signal, known as a spreading code, before transmission. The modulation of the data signals spreads the spectrum of the signals and makes them appear like noise to an ordinary receiver. However, when the same pseudo-random signal is used to demodulate (despread) the transmitted data signal at the CDMA receiver, the data signal can be easily recovered.
An additional advantage of CDMA is that data signals from multiple users can be transmitted simultaneously on the same frequency band. Users are differentiated from one another at the CDMA receiver by spreading codes. Correlators provided at the receiver of the CDMA system recognize each different spreading code and restore (despread) the original data signal. These correlators are often arranged into units called xe2x80x9crakesxe2x80x9d or xe2x80x9crake fingers,xe2x80x9d the function of which is to assemble and demodulate one received multipath propagated signal component. Each rake finger typically includes one or more correlators and one or more pseudo-random code generator associated with each correlator, each pseudo-random code generator being associated with one correlator. The multiple rake fingers are used to detect the strongest multipath components as described below. Each correlator detects a time shifted version of the original transmitted data signal.
One problem that CDMA and other similar transmission schemes must deal with is multipath propagation. Multipath propagation is a phenomenon which causes many different versions of a transmitted signal, called multipath components, to be created and propagated to a receiver. The multipath components are created because the transmission antenna radiates the data signal in many directions (omnidirectionally), and therefore creates more than one component. Each of these different multipath components may arrive at the receiver at different times due to delays created by obstacles in the respective transmission paths. In other words, since the data signals are transmitted over the airspace between the transmitters and receivers, the transmitted signals will incur delays due to the surrounding environment (e.g. they will bounce off buildings and other structures in the transmission path). Hence, the same data signal may arrive at the receiver (or receivers) at different times. Depending on the environment, the multipath components may combine with each other constructively or destructively. Destructive combination causes the multipath components to effectively cancel each other out. Thus, if the multipath components combine with each other destructively, portions of, or the entire data signal may lost.
CDMA systems deal with the problem of multipath propagation by providing multiple antennas to receive the signal and multiple rake fingers. A receiver that includes multiple reception antennae is often referred to as a diversity receiver. Typically, both antennas will not experience the destructive combination of multipath components simultaneously and each rake finger can handle one multipath component from one antenna to overcome the time delays. For example, a CDMA receiver may have three rake fingers dedicated to the transmissions of a single user, to accommodate three different multipath components. Each one of the rake fingers receives a different multipath portion of the transmitted signal from one of the receiving antennas, each portion having a different associated time delay. The CDMA receiver is fabricated to recognize the different time delays and coordinate the multipath signals so that the original data signal can be retrieved from the multipath components.
FIG. 1 shows an example of a wireless system utilizing such a CDMA architecture and transmission scheme. There is shown a base station 100 and multiple user stations 110 and 120 (e.g. wireless phones). Each user station 110, 120 has an associated transmitter 140, 150, for transmitting signals to the base station 100. The base station 100 includes two receiving antennae A and B, connected to a receiver 130 for receiving signals from the multiple user stations 110, 120. As previously noted such a receiver, including multiple reception antennae (i.e. A and B), is often referred to as a diversity receiver. Connected to each user transmitter 140, 150 are transmission antennae C, D respectively. Each user station 110, 120 transmits data signals (i.e. messages) over their respective transmission antennae C, D, to the reception antennae A, B, and through to receiver 130. The base station 100 receives the transmitted data signals, and relays the data (e.g. messages) to other users of the system.
As stated previously, each data signal may follow different paths to the base station receiver 130. For example, these paths are shown as transmission paths P1A and P2A from user station 110, and paths P1B and P2B from user station 120. Although only two multipath components are shown for each user station, any number of multipath components may exist for each transmitted data signal. In most cases, it should be noted that only a few multipath components are dominant (i.e. only a few multipath components are worth considering due to the weak signal strength of the various other non-dominant components). Further, although the multipaths are only shown for transmission from a user station 110,120 to the base station 100, it should be noted that the same multipath components are present when transmitting data from the base station 100 to the individual user stations 110, 120, etc. Therefore, the receivers located at the user stations 110, 120 must also include rake fingers for handling multipath components.
As stated above, in present CDMA receivers, each rake finger includes at least one correlator and a separate pseudo-random code generator for each correlator. In systems with multiple receiving antennae at the base station, such as antennae A and B in FIG. 1, the multiplicity of rake fingers creates a problem in that a large number of code generators are required (i.e., one for each correlator of each antenna).
FIG. 2 shows the traditional rake finger 200 for a dual-antenna diversity receiver. As can be seen, the rake finger 200 includes first and second code generators 210, 220, and first and second correlators 230 and 240. Multiplexers 250, 260 connect the two antennae (e.g. A and B) to each correlator 230, 240. The multiplexers 250, 260 are controlled by control signals 235, 245 which select the antenna and sector of the cell from which the rake finger is currently receiving data signals. Although the multiplexers 250, 260 allow the correlators 230, 240 to receive signals from either antenna, for purposes of this discussion it will be assumed that correlator 230 receives data signals from antenna A only, and correlator 240 receives signals from antenna B only. Thereby, the rake finger 200 forms a single rake finger unit for reception of one multipath signal on both antennas. The base station receiver 130 shown in FIG. 1 includes multiple rake fingers 200. Although the device shown in FIG. 2 is referred to above as a single rake finger, it may also comprise two separate rake fingers, one for the code generator 210, correlator 230, and multiplexer 250, and one for the code generator 220, correlator 240 and multiplexer 260.
FIG. 5 shows another conventional rake finger 400. Rake finger 400 is similar in many respects to rake finger 200 shown in FIG. 2, except that it is configured for a quad-antenna system (i.e. 4 antennae) instead of a dual-antenna system. Rake finger 400 also includes beamforming inputs 401-404, which assist in dividing the antenna coverage into sectors. The beamforming inputs 401-404 restrict the signals output from the correlators 430-445 to those of a single antenna (e.g. antenna A) and a single sector (e.g. sector 2). Rake finger 400 is configured for use with the transmission scheme shown in FIG. 1A, which includes a receiver 130xe2x80x2 with four antennae A-D. The remainder of the transmission system is essentially the same as the system described above with reference to FIG. 1, where similar reference numerals indicate similar parts. Rake finger 400 includes first through fourth code generators 410-425, and first through fourth correlators 430-445. Multiplexers 450-465 connect each antennae (e.g. A-D) to each respective correlator 430-445. The multiplexers 450-465 are controlled by control signals 470-485 which select the antenna and sector of the cell from which the rake finger is currently receiving data signals. Although the multiplexers 450-465 allow the correlators 430-445 to receive signals from any of the antennae, for purposes of this discussion it will be assumed that correlator 430 receives data signals from antenna A only, correlator 435 receives signals from only antenna B, correlator 440 receives signals from only antenna C, and correlator 445 receives signals from only antenna D. Thereby, the rake finger 400 forms a single unit for reception of one multipath signal on all four antennae. As stated above with reference to FIG. 2, the base station receiver 130 shown in FIG. 1 typically include multiple rake fingers 400. Although the device shown in FIG. 5 is referred to above as a single rake finger, it may also comprise four separate rake fingers, one for each antenna A-D. When the device comprises four separate rake fingers, each rake finger includes a code generator, a correlator, and a multiplexer.
The number of rake fingers 200 (or 400) in the receiver 130 is dependent on the number of users of the system, the number of antennae, and the number of multipath components which are used to demodulate the transmitted signal. For example, in a dual-antenna (quad-antenna) system if there were five users, with each user having one rake finger 200 (400) dedicated to his or her multipath components, the receiver would require five rake fingers 200 (400), and therefore ten (twenty) code generators. In most CDMA receivers, however, each user has at least three rake fingers assigned to him or her, thereby pushing the totals up to fifteen rake fingers and thirty (sixty) code generators. This per receiver multiplicity of rake fingers and code generators causes the cost of each CDMA receiver to be relatively high. Additionally, the size, power usage and complexity of the receiver is necessarily increased. Hence, there is presently a need for a base station receiver structure which accommodates multiple antennae, but which also does not similarly increase the size, cost, and complexity of the receiver circuitry.
In view of the foregoing defects, and for other reasons, the present invention is directed to a CDMA system which reduces the size and cost of present CDMA base station receiver by utilizing a single code generator for a set of antennae and accommodating delays present in multiple antenna receivers.
An object of the present invention is to provide an apparatus for providing a digital radio link between at least one fixed unit and at least one mobile unit, where the fixed unit has at least two antennae located at the fixed unit. A receiver is connected to the two antennae and includes at least one multiple rake finger. The multiple rake finger comprises a code generator and at least two correlators corresponding to the two antennae, wherein the code generator provides pseudo-random signals to both of the correlators.
Another object of the invention is to provide a method of operating a digital transmission system comprising the steps of receiving a signal on at least two different antennae, transmitting the signal to at least two correlators, each correlator corresponding to a different antenna, and transmitting a pseudo-random code to the two correlators, wherein the code is generated by a single code generator.
Another object of the invention is to provide a multiple access digital receiver comprising at least one multiple rake finger, the multiple rake finger comprising a code generator and at least two correlators, wherein the code generator provides pseudo-random signals to both of the correlators.
Another object of the invention is to provide a searcher for use with a multiple access digital communication system comprising at least one multiple rake finger, the multiple rake finger comprising a code generator and at least two correlators, wherein the code generator provides pseudo-random signals to both of the correlators.
To accomplish the above objects there is described a multiple rake finger which provides a single code generator for a set of correlators, each correlator corresponding to a separate antenna. The multiple rake finger includes delay circuitry located between the code generator and the correlators for coordinating the code signal with the received signal. The delay circuitry also compensates for the time delay between the antennae.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings and claims.